Xylan is a component of plant hemicellulose. Xylan consists of a backbone of 1,4-glycosidically linked .beta.-D-xylose. Usually xylans have side chains or groups comprising xylose and other pentoses, hexoses, uronic acids and acetyl groups.
In the paper production process the bleaching of pulp is an important step. Schematically, the process used for pulp treatment in the paper and pulp industry is performed as follows: Pulp is treated at pH 10-12 at 80.degree. C. to remove most of the lignin in the so-called oxygen delignification step. The remaining pulp contains 2-5% of lignin. This lignin gives the pulp its brown color. Subsequently, the pulp is bleached in a multistage bleaching process. In this bleaching process chemicals such as chlorine, chlorine dioxide, hydrogen peroxide and/or ozone are used, to obtain a bright pulp for high quality paper.
Chlorine and chlorine-containing chemicals are often used in the bleaching process. However, since the use of these chemicals leads to the formation of dioxin and other chlorinated organic compounds, they form a threat to the environment. Therefore, there is a growing tendency to omit the use of chemicals giving rise to this kind of waste products.
This has prompted a tendency to develop chlorine-free processes; total chlorine-free (TCF) and elementary chlorine-free (ECF). In these processes hydrogen peroxide or ozone is used for bleaching. However, the use of these oxidative chemicals may have a negative effect on the quality of the paper, especially the strength of the paper
It has been found that certain enzymes make the pulp more accessible to bleaching agents, thereby reducing the amount of bleach. Xylanases, in particular, have been found to be very useful in the paper and pulp processing. Xylanases have been reported to increase the extractability of lignins from the pulp. In current processes, xylanases are mostly used after the oxygen delignification step, because they are not active and do not survive under conditions used during oxygen delignification.
Xylanases cleave the hemicellulose chains which are responsible for the close adherence of lignin to the cellulose network. After xylanase treatment the lignin is more easily removed in the subsequent steps.
Therefore the use of xylanases leads to a reduction of the consumption of active chlorine in prebleaching of 25-30%. This reduction of chlorine does not afflict the quality parameters of the resulting paper (Viikari et al. 1986. Proc. of the third Int. Conf. Biotechnology in Pulp and Paper Ind., Stockholm, p.67-69 and Bajpai and Bajpai. 1992. Process Biochemistry. 27:319-325).
The xylanase treatment also reduces the need for other chemicals, such as hydrogen peroxide, in the bleaching process.
The use of xylanases from fungal sources in bleaching of kraft pulp has been reported. These are acidic xylanases and the pH and temperature optima of the enzymes are: pH 3-5 and T 30-50.degree. C. These values are not ideal for the use in the bleaching process, where the prevailing conditions; are pH.gtoreq.9 and temperature .gtoreq.70.degree. C.
Xylanases from bacterial origin, with higher pH and/or temperature optima, have been reported for use in the bleaching process. Some of these originate from the following species (pH and temperature optima of the reported xylanase activity between brackets):
Bacillus pumilus (pH=7-9, T =40.degree. C., Nissen et al. 1992. Progress in Biotechnology 7:325-337), Bacillus stearothermoohilus (pH=7, T=65.degree. C. International patent application WO 91/10724), Dictyoglomus thermophiluim (pH=6-8, T=70.degree. C., European patent application EP 0 511 933), Rhodothermus (pH=5.5-6.5, T=80-100.degree. C., European patent application EP 0 538 177), Thermotoga (pH=5-6, T=90.degree. C., International patent application WO 93/19171) and Thermoanaerobacter ethanolicus (T=68.degree. C., Deblois and Wiegel. 1992. Progress in Biotechnology 7:487-490).
Although the use of some of these xylanases for bleaching has been claimed, no xylanases have been reported to date having the desired characteristics of significant delignification activity at temperature .gtoreq.80.degree. C.